The present invention relates generally to non-contact strain extensometers for mechanical testing, including testing of tensile, flexural, compression, shear, tear, fatigue and fracture values. In particular, the invention relates to a system which employs a laser or light beam to scan the separation of benchmarks on a test specimen. Non-contact extensometers are ideally suited for robot-automated materials testing where conventional contact extensometers cannot be easily used.
Contact extensometers using linear variable differential transformers (LVDTs), strain gages or rotary potentiometers have been available for many years. These devices have the following deficiencies:
(i) They require manual dexterity to clamp on and remove from the test specimen. If not mounted properly, they tend to slip and produce erroneous data.
(ii) Strain gage/LVDT extensometers have limited measurement range. They cannot be used to measure high elongations in elastomeric type materials, or some of the new materials which have high strengths and high elongation.
(iii) Clamp-on, "pogo" extensometers can measure high elongations, but are not accurate for measuring low elongations as found in high strength materials.
(iv) Contact extensometers cannot readily be used in conjunction with robot automated testing systems which operate without any human involvement.
To overcome some of these problems, non-contact strain extensometers have been proposed and developed. A good example is U.S. Pat. No. 4,031,746 (Furuta et al) which discloses a non-contact, optical extensometer. This device uses two light sources reflecting from benchmarks placed on the specimen under test. The reflected light from the two benchmarks is sensed by sensors mounted on servo-motor controlled trackers. As the benchmarks separate during test the servomotor drives the trackers to follow the benchmark and measure the strain. Optical, non-contact extensometers of this type have the following disadvantages:
(i) The accuracy of measurement depends upon the mechanical motion and precision of the servo-controllers. While these accuracies are reasonable for high elongation materials, they cannot achieve the +/- 0.5% accuracy required for high strength materials. The Furuta et al patent, for example, discloses measurement of only the elongation at break, and not the strain in the critical modulus region where higher accuracies are required.
(ii) Some optical extensometers require manual focusing of each sensor on the benchmarks prior to testing. Those which do not require focusing have to provide a mechanism of "hunting" for the benchmarks and locking onto them.
(iii) Mechanical tracking devices suffer from error-producing backlash, high maintenance and wear-and-tear problems. In addition they tend to be bulky devices because of the number of mechanical components required.
A variety of techniques have been described by Petrohilos (U.S. Pat. Nos. 3,765,774; 3,905,705; and 4,007,992), Zanoni (U.S. Pat. No. 3,853,406) and others for using lasers to scan an object to measure its dimensions. These techniques share the following characteristics:
(i) A rotating polygon or mirror is used to produce a revolving beam. This beam is converted into a parallel beam by using optical lenses. However, the object to be measured must necessarily be less than the size of the lens or else it will be outside the scan of the laser.
(ii) The object whose dimensions are to be measured is placed in the path of the parallel beam, thus obstructing the transfer of energy to a sensor on the other side. The duration of the obstruction is measured by timers to obtain measure of the dimension.
These non-contact laser gages cannot readily be used in mechanical testing for the measurement of strain, for the following reasons:
(i) The specimen being tested is typically of a "dogbone" shape. Thus, in contrast to dimensional measurement settings, the specimen obstructs the entire scan.
(ii) Benchmarks are marked on the specimen under test and it is necessary to measure the reflected energy rather than obstructed energy.
(iii) The separation of the benchmark varies from one (1) inch at the start of the test to up to 20 inches at the end of the test. A parallel beam of such a size is not practical.
The prior art also contains bar code readers which use laser scanner and the sensing of reflected energy. These systems are not used for measurement of objects, but instead for decoding series of parallel bars printed on a product's package. The sequence of varying width bars is typically not longer than two inches, with bar lengths of under one inch. Conventional bar code reader configurations are not readily used to provide accurate measurement of objects.